This invention relates to ion sources that are suitable for use in ion implanters and, more particularly, to ion sources for generating oxygen ions.
An ion source is a critical component of an ion implanter. The ion source generates an ion beam which passes through the beamline of the ion implanter and is delivered to a semiconductor wafer. The ion source is required to generate a stable, well-defined beam for a variety of different ion species and extraction voltages. In a semiconductor production facility, the ion implanter, including the ion source, is required to operate for extended periods without the need for maintenance or repair. In addition, the ion source is required to generate a relatively high current ion beam so that implant times to achieve a given dose are kept as short as possible.
In applications which require implantation of oxygen ions, oxygen must be ionized in the ion source. Ordinary oxygen-bearing gases create an oxidizing environment for the refractory materials that are used in the ion source arc chamber. Consequently, these refractory materials, typically tungsten and molybdenum, are oxidized during operation of the ion source for oxygen implantation. The hot tungsten filament or cathode is consumed at an accelerated rate due to oxidation, resulting in extremely short source life. Tungsten and molybdenum oxides, which are gaseous and which are more stable at high temperatures, vaporize and deposit metallic tungsten or molybdenum on arc chamber components that operate at lower temperatures. These deposits accumulate and may create electrical shorts across normally insulating components. Such electrical shorts further reduce the service life of the ion source.
Prior art ion sources have used CO2, CO, N2O, NO and NO2 gases, instead of O2, to lower the oxidizing potential. Carbonaceous gases, such as CO2 and CO, result in the deposition of carbon or graphite on the components of the arc chamber, thereby creating electrical shorts that reduce the service life of the ion source.
All of the known prior art oxygen ion sources have had one or more disadvantages, including short operating life and deposition of conductive materials on components of the arc chamber. Accordingly, there is a need for improved methods and apparatus for generating oxygen ions.
In accordance with an aspect of the invention, water vapor (gaseous H2O) is used as a source feed material for the implantation of oxygen. In order to prevent the rapid oxidation of components inside the arc chamber in the presence of an oxygen-bearing gas, the free energy of formation of the gas must be lower than the free energy of formation of the oxides of the tungsten filament and of any other refractory metals in the arc chamber, typically tungsten or molybdenum, for most of the temperature range of operation of the source. The minimum free energy state of the system is that which keeps the oxygen-bearing gas concentration maximum, thereby preventing or slowing the oxidation of the refractory metals. Problems of short source life and electrically shorting deposits, normally caused by the presence of an oxygen-bearing gas, are avoided or minimized.
According to another aspect of the invention, a method is provided for generating oxygen ions in an ion source including an arc chamber containing at least one oxidizable metal. The method comprises the steps of feeding gaseous H2O into the arc chamber and operating the arc chamber in a temperature range where the free energy of formation of gaseous H2O is less than the free energy of formation of oxides of the oxidizable metal.
In one embodiment, the metal comprises tungsten and the arc chamber is operated in a temperature range of about 960xc2x0 C. to about 3300xc2x0 C. In another embodiment, the metal comprises molybdenum and the arc chamber is operated in a temperature range of about 1050xc2x0 C. to about 2800xc2x0 C. In a further embodiment, the metal comprises rhenium, and temperature control is not required to limit oxidation.
The temperature of the arc chamber may be controlled within the specified temperature range. In one embodiment, the temperature is controlled by thermally shielding the arc chamber.
According to a further aspect of the invention, an oxygen ion source is provided. The oxygen ion source comprises an arc chamber containing a cathode, a source of gaseous H2O coupled to the arc chamber for introducing gaseous H2O into the arc chamber, and an energy source for producing a plasma in the arc chamber.
The ion source may further comprise a temperature control device for operating the arc chamber in a temperature range where the free energy of formation of gaseous H2O is less than the free energy of formation of oxides of oxidizable metals located in the arc chamber.
The arc chamber may be fabricated of tungsten, molybdenum, rhenium or alloys containing these metals, and the cathode may be fabricated of tungsten, molybdenum, rhenium or alloys containing these metals.